Lecture 15 - Cytoskeleton & CM: Microtubules Flashcards
what are the 5 roles of the cytoskeleton?
- scaffolding that provides structural support
- network of tracks to direct the movement of material
- force generating apparatus for movement and contraction
- a framework for positioning various organelles within the cell
- essential component of the cell division machinery
__________ are the largest of the cytoskeletal components of the cell
microtubules
what are the 2 types of microtubules in the cell?
- cytoplasmic microtubules
- axonemal microtubules
what are cytoplasmic microtubules responsible for?
- maintaining axons
- formation of mitotic and meiotic spindles
- placement and movement of vesicles
- maintaining or altering cell shape
what are axonemal microtubules responsible for?
- cilia
-flagella - basal bodies to which cilia and flagella attach
def: the central shaft of a cilium or flagellum, is a highly ordered bundle of MT’s
axoneme
what are the protein building blocks of microtubules?
tubulin heterodimers
MT’s usually consist of 13 longitudinal arrays of polymers called ___________
protofilaments
what is the basic subunit of a protofilament?
heterodimer of tubulin, one alpha-tubulin and one beta-tubulin
bind covalently to form alpha-beta- heterodimer
T or F: all the dimers in the MT are oriented the same way
true
as a result of their dimer orientation, all protofilaments have _______ _________
inherent polarity
cytoplasmic MTs are _____ ______ with 13 protofilaments?
simple tubes
some axonemal MTs form _______ or ________ MTs
doublet or triplet
where can doublet MTs be found
in cilia and flagella
where can triplet MTs be found
in basal bodies and centrioles
when does reversible polymerization of tubulin dimers occur?
only in the presence of GTP and Mg2+
def: dimers aggregate into oligomers, which serve as “nuclei” from which new MTs grow
nucleation
def: the addition of more subunits at either end
elongation
how do the lag phase and elongation phase compare
the lag phase starts off slower since nucleation is a slow process but then the elongation phase is much faster
def: when the concentration of tubulin becomes limiting, the assembly is balanced by disassembly
plateau phase
def: concentration in which MT assembly is exactly balanced by disassembly
critical concentration
where does the addition of tubulin occur quicker?
the plus ends of the microtubules
T or F: the plus and minus ends of microtubules have the same critical concentation
false, they differ
def: addition of subunits at the plus end, and removal from the minus end
treadmilling
if [tubulin subunits] is above critical concentration for the plus end but below the minus end, _________ will occur
treadmilling
when is the GTP bound to the submit hydrolyzed to GDP?
after the heterodimer is added to the MT
is GTP hydrolysis required for MT assembly?
no, it is needed to promote interactions and addition to MTs
def: one population of MTs grows by polymerization at the plus ends whereas another population shrinks by de-polymerization
dynamic instability model
the GTP cap at the plus end prevents _________ __________
subunit removal
growing MTs have _____ at the plus ends, and shrinking MTs have ______
GTP, GDP
hydrolysis of GTP after assembly changes the conformation of the subunits and tends to force the protofilament into…….
a curved shape that is less able to pack into the microtubule wall
which end of the microtubule has a higher critical concentration and therefore will grow slower?
the negative end, the plus end will grow faster at the same concentrations
def: where MTs originate from, serves as a site of MT assembly nucleation and acts as an anchor for one end(minus)
microtubule-organizing centre (MTOC)
an example of an MTOC in cells during interphase is
centrosome near the nucleus
centriole walls are formed by ___ ____ ___ ________ and are oriented at ___ ____ to each other
9 sets of triplet microtubules, right angles
can cells divide without centrioles?
yes, but they have poorly organized mitotic spindles
where are gamma-tubulins found ?
only in centrosomes
def: nucleate the assembly of new MTs away from the centrosome
gamma- tubulin ring complexes (gamma-TuRCs)
what does loss of gamma-TuRCs prevent?
prevents the cell from nucleating MTs
what is the most important role of MTOCs?
the ability to nucleate and anchor MTs
which end of the MTs are anchored in the MTOC
the minus end
what are the 4 ways that cells control microtubule assembly and structure?
- MT-binding proteins that use ATP to drive vesicle or organelle transport or generate sliding forces between MTs
- Microtubule-Stabilizing/Bundling Proteins
- Plus-End Tubulin Interacting Proteins
- Microtubule-Destabilizing/Severing Proteins
def: bind at regular intervals along a microtubule wall, allowing for interaction with other cellular structures and filaments
Microtubule-Stabiliziing/Bundling proteins
def: MAP that causes MTs to form tight bundles in axons
Tau
def: MAP that promotes the formation of looser bundles in dendrites
MAP2
what part of the MAP controls the spacing of MTs in the bundle?
the extended arm
can MTs stay intact for long periods of time?
no, they are too unstable, and will de-polymerize unless they are stabilized in some way
what can stabilize MTs?
+-TIPs capture the growing plus end and protect it form catastrophic subunit loss
def: bind to tubulin heterodimers and prevent their polymerization
Stathmin/Op18
def: act as the ends of microtubules, promoting the peeling of subunits from the ends
catastrophins
def: proteins that sever MTs
katanins
